Fuel cells for combining hydrogen and oxygen to produce electricity are well known. A known class of fuel cells includes a solid-oxide electrolyte layer through which oxygen anions migrate; such fuel cells are referred to in the art as “solid-oxide” fuel cells (SOFCs).
In some applications, for example, as an auxiliary power unit (APU) for a transportation application or a stationary power unit (SPU) for a stationary application, an SOFC is preferably fueled by “reformate” gas, which is the effluent from a catalytic liquid or gaseous hydrocarbon oxidizing reformer, also referred to herein as “fuel gas”.
Reformate typically includes amounts of carbon monoxide (CO) as fuel in addition to molecular hydrogen. The reforming operation and the fuel cell operation may be considered as first and second oxidative steps of the hydrocarbon fuel, resulting ultimately in water and carbon dioxide. Both reactions are preferably carried out at relatively high temperatures, for example, in the range of 700° C. or higher.
A complete fuel cell stack assembly includes a plurality of fuel cells, for example, 60 cells in the form of sub-assemblies which are electrically connected in series. Typically, connection of adjacent fuel cell sub-assemblies comprises a separator plate having a conductive interconnect element on each side, the interconnects being disposed in the fuel gas and air flow spaces and in electrical contact with the anodes and cathodes of the fuel cells.
Maintaining good electrical contact between a separator plate and fuel cell electrodes is essential in operating an SOFC stack with high efficiency. In the prior art, a conductive metal mesh is attached on a first side to the separator plate via a plurality of spot welds and the other side of the mesh makes contact with the fuel cell electrode via mechanical contact. To further ensure electron flow into and out of the fuel cell, a conductive material is applied in a grid pattern to the anode surface of the electrode and sintered to ensure bonding and adequate porosity, and the interconnect mesh is bonded to the conductive material. The bond joints are made by sintering a paste containing nickel oxide (NiO) particles onto the anode surface. In the prior art, the grid pattern also is formed by a paste. A serious drawback is that in the event the grid is fired at too high a temperature or not formulated properly, the grid pattern could become densified and non-porous, preventing the anode reactive gas from sufficiently penetrating the porous microstructure, starving the reaction zone of fuel supply. This would lower the efficiency of the electrochemical reaction at the triple phase boundary and reduce cell performance.
The SOFC anode layer of the electrode is a cermet comprising nickel oxide and yttria-stabilized zirconia (YSZ). A contact paste used on the anode surface has a few basic requirements. The paste must provide and maintain electrical conductivity between the fuel cell anode and the interconnect in the anode operating environment (temperature, reducing atmosphere, coefficient of thermal expansion, and the like). It is vital that good adhesion of the paste to the interconnect and to the fuel cell anode is obtained so that this electrical path is maintained throughout the life of the fuel cell stack, after many thermal/electrical cycles. Also, it is highly desirable that the contact paste, after sintering, be sufficiently porous to allow the flow of fuel gas into, and by-products out of, the anode. In the prior art, the contact paste spots can become densified and non-porous and thus represent areas where the anode reaction is prevented from taking place or at the least reduces the diffusion of the reactant gas through the microstructure, limiting the availability of the reactive species at the triple phase boundary. Reduction of the permeability of fuel gas through the electrode microstructure is believed to be a major contributor to reduced fuel utilization.
A prior art method for forming the electrical connections of the interconnect to the separator plate and to the anode surface of an SOFC comprises the steps of:
a) fabricating a sintered fuel cell bi-layer comprising the electrolyte layer on the anode support;
b) screen printing a paste containing NiO in a screen (grid) pattern onto the anode surface;
c) sintering the screened pattern at 1200° C.;
d) printing the cathode material onto the electrolyte layer and sintering the cathode;
e) spot welding a first side of an interconnect on the anode side to a separator plate at a plurality of locations;
f) applying anode contact paste to the anode grid pattern at a plurality of locations;
g) positioning the separator plate such that the second side of the interconnect is in contact with the anode contact paste;
h) sintering the anode contact paste in air to 825° C., then reducing the stack operating temperature to 750° C.; and
i) exposing the anode side of the cell to a reducing gas to reduce the NiO to Ni in the anode itself, the anode grid, and the anode contact paste.
Anode contacts formed in accordance with the prior art method and materials are vulnerable to at least two serious and known shortcomings.
First, adhesion of the interconnect to the anode surface is typically rather poor and results in significant loss of power due to interfacial contact failures. It has been found that a large part of the adhesion problem is due to low bond strength between the sintered grid and the anode surface. Further, the paste does not bond especially well with the sintered grid.
Second, the NiO in the grid and in the contact paste sinters as NiO but is then reduced by the fuel gas to metallic Ni, and becomes very dense and non-porous. This is detrimental as the availability of the anode surface to reactant gas is reduced. Closing off the open pore volume inherent in the pre-sintered paste to reactant gas flow may cause a reduction in cell operating voltage due to concentration polarization as well as substantial reduction in fuel utilization. This also can interfere with reactant by-products such as water and carbon dioxide escaping from the cell, leading to critical cell failure.
A similar situation can occur on the cathode side of the fuel cell. In this case, due to the oxidizing environment, a noble metal (or alloy) is preferably used to form a contact grid on the cathode. A cathode contact paste, containing noble metal (or alloy) particles, can then be used to make a bonded electrical contact joint from the cathode to the interconnect on the cathode side of the cell. The cathode contact formed in accordance with the prior art method and materials is vulnerable to the paste particles sintering to form non-porous areas covering the cathode. This phenomenon can result in reduced oxygen transport to the electrolyte which leads to diminished cell performance.
What is needed in the art is an improved anode and cathode electrical connection that utilizes an improved interconnect to electrode contact paste.
What is further needed in an improved method of cell anode and cathode contact surface fabrication.
It is a principal object of the present invention to increase the reliability and longevity of an SOFC stack.